The present invention relates to an optical recording/reproducing apparatus which uses an optical data recording medium such as an optical disk.
In general, in order to read out data from an optical disk, a laser beam is emitted onto a data recording track (hereinafter referred to as a track), and data is reproduced in accordance with the beam reflected by the disk or transmitted through the disk. When tracks are helically formed on the optical disk, servo control (tracking control) is necessary for an optical head to accurately emit the reproduction laser beam onto a given track, since sectors of the same track are not equidistantly arranged with respect to the center of rotation. Even if tracks are concentrically formed on the optical disk, sectors within the same track may not be equidistantly arranged with respect to the center of rotation due to eccentricity of the disk or the like, thus making tracking control necessary.
A conventional tracking circuit is available which changes the position of an objective lens in response to a tracking error signal obtained by light reflected by or transmitted through an optical disk. When a track error caused by eccentricity exceeds 30 to 40 .mu.m, however, the objective lens itself deviates considerably from the mechanical center. In this case, an optical offset signal is superposed on the tracking error signal. Therefore, the laser beam scans an erroneous track in response to the optical offset signal.
In order to eliminate the optical offset signal, a conventional two-stage servo control system has been developed (e.g., Japanese Patent Disclosure No. 59-152572). According to this system, an optical head carriage is driven in addition to an objective lens, with tracking control being performed in combination by the objective lens and the optical head carriage. More specifically, the tracking error signal is supplied not only to an objective lens drive coil but also to a carriage drive voice-coil motor.
The conventional two-stage servo control system also has a drawback. Since the relationship between the drive timings of an object lens and a carriage is not fixed, it takes a long period of time to stabilize a laser beam on a track if the timings are not properly set.